The present invention relates to methods of making improved semiconductor wafers with heat spreading layers of isotopically pure semiconductor materials and the wafers derived from these methods.
Silicon, germanium, and gallium compounds are commonly used to fabricate semiconductor wafers. In the naturally occurring forms, silicon is composed of approximately 92.2% 28Si, 4.7% 29Si, and 3.1% 30Si; germanium is composed of approximately 20.5% 70Ge, 27.4% 72Ge, 7.8% 73Ge, 36.5% 74Ge, and 7.8% 76Ge; and gallium is composed of approximately 60.4% 69Ga and 39.6% 71Ga, which are roughly the composition of crystals used by the semiconductor industry. However, semiconductor devices composed of naturally occurring silicon, germanium, and gallium have properties such as carrier mobility which place limits upon the semiconductor speed since carrier mobility governs signal transit times in semiconductor materials. Power dissipation in a semiconductor is limited by the thermal conductivity of the materials from which it is made. This thermal conductivity in turn limits the packing density of the transistors on a semiconductor wafer or the amount of power that can be generated in a device without inducing device failure.
In an effort to reduce the price for gallium arsenide or gallium nitride based devices, investigation into the uses of other substrate materials such as germanium, sapphire and silicon carbide have been investigated. Recently several organizations have been successful in using silicon as a substrate material to grow thin film devices based on gallium compounds.
One of the limiting factors in the lifetimes of thin film devices is the power output, i.e., high power devices such as semiconductor lasers and LED""s have shorter lifetimes when operated at higher power because higher temperatures are generated causing degradation of the materials. Many devices use external cooling methods such as copper heatsinks to limit the temperature rise of the device, but further improvements in cooling technology are necessary to allow these devices to be used in higher brightness or high power applications.
Semiconductor wafers with increased thermal conductivity will allow for increased power densities in these devices, thereby enhancing the performance of many electronic devices now on the market.
Accordingly, the present invention is directed to wafer structures having increased thermal conductivity over conventional semiconductor wafer designs. In one embodiment, the invention provides a wafer having a layer of an isotopically-enriched material on at least one surface of the substrate. The isotopically-enriched material may be isotopically-enriched silicon, germanium, silicon-germanium alloys, gallium arsenide, aluminum gallium arsenide, gallium nitride, gallium phosphide, gallium indium nitride, indium phosphide or combinations and alloys of these materials.
In another embodiment, the wafer structure includes an additional top semiconductor layer formed on the layer of isotopically-enriched material. Optionally, a semiconductor device is formed in the top semiconductor layer.
The invention provides methods of making these wafers including methods of removing the substrate from the wafer to leave a top semiconductor layer on a layer of isotopically-enriched materials with no underlying substrate.
In one embodiment, the wafer structure comprises silicon enriched to at least 98% 28Si, and the semiconductor layer comprises silicon having naturally-occurring isotopic ratios.